The study will determine whether: 1) a reduction in regional cerebral oxygen availability (detected by NIRS) signaling impending cerebral ischemia correlates with a reduction in CBF as measured by either bedside, radioactive 133Xe or stable Xe-enhanced CT and with other indices indicative of reduced cerebral perfusion such as increased ICP or reduced CPP; 2) an increased cerebral oxygen availability (as detected by NIRS) indicates cerebrovascular dilation and vasoparalysis. A reduction in CBV indicates cerebral edema in the genesis of intracranial hypertension; 3) application of appropriate therapy with successful reduction in ICP and increase in CPP correlates with increased cerebral availability.